Composite substrate and manufacturing method thereof

ABSTRACT

A supporting substrate and a piezoelectric substrate are prepared. A surface of the supporting substrate is bonded to the backside of the piezoelectric substrate with an organic adhesive layer interposed therebetween to form a laminated substrate. Subsequently, a peripheral surface of the laminated substrate is ground such that a peripheral surface of the piezoelectric substrate, a peripheral surface of the organic adhesive layer, and a peripheral surface of the supporting substrate on the side of the organic adhesive layer are made flush with each other. Subsequently, the surface of the piezoelectric substrate is polished to reduce the thickness of the piezoelectric substrate and performing mirror polishing of the surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a compositesubstrate and to a composite substrate.

2. Background of the Invention

It is already known that a composite substrate, in which a supportingsubstrate and a piezoelectric substrate are bonded together, is providedwith electrodes to fabricate elastic wave devices. For example, aselastic wave devices, surface acoustic wave devices and bulk acousticwave devices have been used as band-pass filters in communicationdevices, such as mobile phones. Composite substrates are known to uselithium niobate or lithium tantalate as a piezoelectric substrate andsilicon or quartz as a supporting substrate (see Patent Document 1).

-   [Patent Document 1] Japanese Unexamined Patent Application    Publication No. 2006-319679

Such composite substrates are often manufactured by preparing apiezoelectric substrate and a supporting substrate, bonding thepiezoelectric substrate to the supporting substrate with an organicadhesive layer interposed therebetween, and reducing the thickness ofthe piezoelectric substrate. Since an edge of a substrate may hitsomething and be broken during handling, a piezoelectric substrate isgenerally beveled. When the thickness of a piezoelectric substrate isdecreased, abrasive grains are placed between a surface of apiezoelectric substrate and a lapping plate, and the surface of thepiezoelectric substrate is lapped with the lapping plate. However,according to such a manufacturing method, when a piezoelectric substrateand a supporting substrate are bonded together, an organic adhesivelayer is not fully applied up to a beveled edge of the piezoelectricsubstrate. Furthermore, since beveling increases the distance betweenthe piezoelectric substrate and the supporting substrate, an edge of thepiezoelectric substrate is not bonded to the supporting substrate. Insuch a state, lapping of the surface of the piezoelectric substrate witha lapping plate causes a problem of frequent occurrence of chipping fromthe edge.

SUMMARY OF THE INVENTION

In view of the problems described above, for a composite substrate inwhich a piezoelectric substrate is bonded to a supporting substrate withan organic adhesive layer interposed therebetween, it is a principalobject of the present invention to prevent chipping of an edge of thepiezoelectric substrate when the surface of the piezoelectric substrateis treated with abrasive grains.

Means for Solving the Problems

To achieve this object, the present invention has employed the followingmeans.

The present invention provides a method for manufacturing a compositesubstrate invention including the steps of:

(a) preparing a supporting substrate and a piezoelectric substratehaving a beveled edge;

(b) bonding a surface of the supporting substrate to the backside of thepiezoelectric substrate with an organic adhesive layer interposedtherebetween to form a laminated substrate;

(c) grinding a peripheral surface of the laminated substrate such that abevel of the piezoelectric substrate is removed and such that aperipheral surface of the piezoelectric substrate, a peripheral surfaceof the organic adhesive layer, and a peripheral surface of thesupporting substrate on the side of the organic adhesive layer are madeflush with each other; and

(d) placing abrasive grains between the surface of the piezoelectricsubstrate and a lapping plate, and lapping the surface of thepiezoelectric substrate with the lapping plate to reduce the thicknessof the piezoelectric substrate and performing mirror polishing of thesurface of the piezoelectric substrate,

wherein the grinding of a peripheral surface of the laminated substratein the step (c) is performed such that the initial outer diameter of thesupporting substrate is maintained.

The present invention provides a composite substrate, including:

a supporting substrate;

a piezoelectric substrate; and

an organic adhesive layer for bonding the piezoelectric substrate to thesupporting substrate,

wherein a peripheral surface of the piezoelectric substrate, aperipheral surface of the organic adhesive layer, and a peripheralsurface of the supporting substrate on the side of the organic adhesivelayer are flush with each other, and

the supporting substrate has an outer diameter larger than the outerdiameter of the peripheral surface of the supporting substrate on theside of the organic adhesive layer.

In accordance with a method for manufacturing a composite substrateaccording to the present invention, when the step (d) is performed, theperipheral surface of the piezoelectric substrate has no bevel and isflush with the peripheral surface of the organic adhesive layer. Thus,an edge of the piezoelectric substrate is bonded to the supportingsubstrate via the organic adhesive layer and is resistant to chipping,as compared with a state in which an edge of the piezoelectric substrateis not bonded to the supporting substrate, for example, because ofbeveling. In a composite substrate in which a piezoelectric substrate isbonded to a supporting substrate with an organic adhesive layerinterposed therebetween, therefore, chipping of an edge of thepiezoelectric substrate can be prevented when the surface of thepiezoelectric substrate is treated with abrasive grains. This isprobably because, when an edge of the piezoelectric substrate is notbonded to the supporting substrate, lapping and polishing of the surfaceof the piezoelectric substrate forms a sharp edge, and force in thethickness direction of the piezoelectric substrate during lapping andpolishing tends to cause chipping, but when an edge of the piezoelectricsubstrate is bonded to the supporting substrate, such chipping rarelyoccurs. Furthermore, in accordance with a method for manufacturing acomposite substrate according to the present invention, after the step(c) is performed, the peripheral surface of the piezoelectric substrate,the peripheral surface of the organic adhesive layer, and the peripheralsurface of the supporting substrate on the side of the organic adhesivelayer are flush with each other. This ensures that no organic adhesivelayer exists outside the peripheral surface of the piezoelectricsubstrate. Thus, in the step (d), when the surface of the piezoelectricsubstrate is polished, this can prevent the organic adhesive layer frombeing detached and attaching as a contaminant to the surface of thepiezoelectric substrate or adversely affecting surface finishing.Furthermore, in accordance with a method for manufacturing a compositesubstrate according to the present invention, since the step (c)involves grinding a peripheral surface of the laminated substrate suchthat the initial outer diameter of the supporting substrate ismaintained, apparatuses and jigs according to the specifications basedon the constant outer diameter can be used before and after the step(c).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a composite substrate manufacturedby a method for manufacturing a composite substrate according to thepresent invention,

FIG. 2 is a fragmentary sectional view of a composite substratemanufactured by a method for manufacturing a composite substrateaccording to the present invention,

FIG. 3 is a fragmentary sectional view of a composite substratemanufactured by a method for manufacturing a composite substrateaccording to the present invention,

FIG. 4 shows schematic cross-sectional views of a manufacturing processof a composite substrate,

FIG. 5 is a fragmentary sectional view illustrating a bevel,

FIG. 6 is a schematic explanatory drawing of the structure of a grindingapparatus 20,

FIG. 7 is an A-A′ fragmentary sectional view illustrating the movementof a grinding wheel 24 in grinding of a peripheral surface of alaminated substrate 16,

FIG. 8 shows freehand drawings illustrating chipping of an edge of an LTsubstrate in an example,

FIG. 9 shows freehand drawings illustrating chipping of an edge of an LTsubstrate in a comparative example,

FIG. 10 is a cross-sectional view illustrating the state after grindingof a peripheral surface of the laminated substrate 16 in a comparativeexample.

DETAILED DESCRIPTION OF THE INVENTION

In a method for manufacturing a composite substrate according to thepresent invention, the step involves preparing a supporting substrateand a piezoelectric substrate that can propagate an elastic wave and hasa beveled edge. The piezoelectric substrate may be a substrate that canpropagate an elastic wave (in particular, a surface acoustic wave).Examples of the material of the piezoelectric substrate include lithiumtantalate, lithium niobate, lithium niobate-lithium tantalate solidsolution single crystal, lithium borate, langasite, and crystal.Although the size of the piezoelectric substrate is not limited, forexample, the diameter ranges from 50 to 150 mm, and the thickness rangesfrom 150 to 500 μm. Furthermore, since the piezoelectric substrate has abeveled edge, even when the edge hits something during the transfer ofthe piezoelectric substrate, the edge rarely chips. Beveling may beC-beveling, in which an intersecting portion of two surfaces (an edge)is cut at a predetermined angle, or R-beveling, in which the edge is cutat a predetermined curvature radius. The backside of the piezoelectricsubstrate may be overlaid with, for example, a metal or silicon dioxidelayer having a thickness in the range of 0.1 to 5 μm. Examples of thematerial of the supporting substrate include silicon, sapphire, aluminumnitride, alumina, borosilicate glass, quartz glass, spinel, lithiumtantalate, lithium niobate, lithium niobate-lithium tantalate solidsolution single crystal, lithium borate, langasite, and crystal.Although the size of the supporting substrate is not limited, forexample, the diameter ranges from 50 to 150 mm, and the thickness rangesfrom 150 to 500 μm. The supporting substrate may have a beveled edge.

In a method for manufacturing a composite substrate according to thepresent invention, the step (b) involves bonding a surface of thesupporting substrate to the backside of the piezoelectric substrate withan organic adhesive layer interposed therebetween to form a laminatedsubstrate. For example, an organic adhesive is uniformly applied to oneor both of the surface of the supporting substrate and the backside ofthe piezoelectric substrate and is solidified while the piezoelectricsubstrate is placed on the supporting substrate to form a laminatedsubstrate. Examples of a method for applying the organic adhesiveinclude spin coating and printing. Examples of the organic adhesiveinclude an epoxy adhesive and an acrylic adhesive. When a supportingsubstrate having a smaller thermal expansion coefficient than thepiezoelectric substrate is prepared in the step (a), the organicadhesive layer formed by heating preferably has a thickness in the rangeof 0.1 to 1.0 μm. Examples of such a case include a case in which thepiezoelectric substrate is formed of a material selected from the groupconsisting of lithium tantalate, lithium niobate, lithiumniobate-lithium tantalate solid solution single crystal, lithium borate,langasite, and crystal and the supporting substrate is formed of amaterial selected from the group consisting of silicon, sapphire,aluminum nitride, alumina, borosilicate glass, and quartz glass. In thatcase, when a surface acoustic wave element is manufactured using afinally manufactured composite substrate, the surface acoustic waveelement can exhibit relatively small variations in frequencycharacteristics with temperature changes. When the organic adhesivelayer has a thickness above 1.0 the organic adhesive layer reduces adifference in thermal expansion coefficient between the piezoelectricsubstrate and the supporting substrate, and unfavorably the effect ofreducing variations in frequency characteristics with temperaturechanges cannot be achieved. When the organic adhesive layer has athickness below 0.1 μm, unfavorably the effect of reducing variations infrequency characteristics with temperature changes cannot be achievedbecause of the effects of voids.

In a method for manufacturing a composite substrate according to thepresent invention, the step (c) involves grinding a peripheral surfaceof the laminated substrate such that a bevel of the piezoelectricsubstrate is removed and such that a peripheral surface of thepiezoelectric substrate, a peripheral surface of the organic adhesivelayer, and a peripheral surface of the supporting substrate on the sideof the organic adhesive layer are made flush with each other. In thegrinding of a peripheral surface of the laminated substrate in the step(c), grinding is performed such that the initial outer diameter of thesupporting substrate is maintained. Examples of a grinding apparatus forgrinding a peripheral surface of the laminated substrate include agrinding apparatus in which a peripheral surface of a rotating laminatedsubstrate is brought into contact with a whetstone of a rotatinggrinding wheel, a grinding apparatus in which a peripheral surface of arotating laminated substrate is brought into contact with a nonrotatingwhetstone, and a grinding apparatus in which a whetstone of a rotatinggrinding wheel that revolves about a laminated substrate is brought intocontact with a peripheral surface of the laminated substrate. In thestep (c), the laminated substrate is ground such that at least part ofthe supporting substrate is ground and such that a peripheral surface ofthe piezoelectric substrate, a peripheral surface of the organicadhesive layer, and a peripheral surface of the supporting substrate onthe side of the organic adhesive layer are made flush with each other.This ensures the removal of a portion of the organic adhesive layerdisposed outside the peripheral surface of the piezoelectric substrate.Consequently, in the subsequent steps, this can prevent the organicadhesive layer from being detached and attaching as a contaminant to thesurface of the piezoelectric substrate or adversely affecting finishingof the surface of the piezoelectric substrate. Furthermore, since theperipheral surface of the laminated substrate is ground such that theinitial outer diameter of the supporting substrate is maintained,apparatuses and jigs according to the specifications based on theconstant outer diameter can be used before and after the step (c).

In a method for manufacturing a composite substrate according to thepresent invention, the step (d) involves placing abrasive grains betweenthe surface of the piezoelectric substrate and a lapping plate, andlapping the surface of the piezoelectric substrate with the lappingplate to reduce the thickness of the piezoelectric substrate andperforming mirror polishing of the surface of the piezoelectricsubstrate. Examples of an apparatus used in the step (d) include generallapper and polisher. For example, with a lapper and polisher for lappingand polishing one side of a laminated substrate, a laminated substrateto be lapped is first placed under pressure between a pressure plate anda lapping plate, and the pressure plate is rotated while slurrycontaining abrasive grains is supplied between the laminated substrateand the lapping plate, thereby reducing the thickness of thepiezoelectric substrate. Subsequently, the lapping plate is replacedwith a lapping plate having a pad on the surface, and the abrasivegrains are replaced with abrasive grains having a higher grit number.The pressure plate is rotated and revolved to perform mirror polishingof the surface of the piezoelectric substrate. Since the step (c)involves grinding a peripheral surface of the laminated substrate suchthat a bevel of the piezoelectric substrate is removed and such that aperipheral surface of the piezoelectric substrate and a peripheralsurface of the organic adhesive layer are made flush with each other, inthe laminated substrate subjected to the step (d), an edge of thepiezoelectric substrate is bonded to the supporting substrate with theorganic adhesive layer interposed therebetween. Thus, in treatment ofthe surface of the piezoelectric substrate with abrasive grains, an edgeof the piezoelectric substrate is resistant to chipping, as comparedwith a substrate an edge of which is not bonded to the supportingsubstrate (for example, the laminated substrate before the step (c)).

FIG. 1 illustrates a cross section of a specific example of a compositesubstrate manufactured by the above-mentioned method for manufacturing acomposite substrate. In the figure, a portion within a dotted linerepresents a portion ground in the step (c), and a portion within analternate long and short dashed line represents a portion removed bylapping and polishing in the step (d). FIG. 1 illustrates a compositesubstrate 7 prepared by grinding the laminated substrate such that aperipheral surface of the piezoelectric substrate 1, a peripheralsurface of the organic adhesive layer 4, and a peripheral surface 2 a ofthe supporting substrate 2 on the side of the organic adhesive layer 4are made flush with each other and polishing the piezoelectric substrate1. A portion within parentheses in FIG. 1 represents the laminatedsubstrate 6 prepared in the step (b). Since the composite substrate 7illustrated in FIG. 1 is manufactured by the above-mentionedmanufacturing method, an edge of the piezoelectric substrate 1 isresistant to chipping. Furthermore, since no organic adhesive layer 4exists outside the peripheral surface of the piezoelectric substrate 1,when the surface of the piezoelectric substrate 1 is polished, theorganic adhesive layer 4 is prevented from being detached and attachingas a contaminant to the surface of the piezoelectric substrate 1 oradversely affecting surface finishing. Furthermore, since the initialouter diameter of the supporting substrate 2 is maintained even afterthe grinding in the step (c), apparatuses and jigs according to thespecifications based on the outer diameter before the grinding in thestep (c) can be used.

In a method for manufacturing a composite substrate according to thepresent invention described above, the angle θ between a plane on theside of the organic adhesive layer 4 in a portion 2 b of the supportingsubstrate 2 having the initial outer diameter and the peripheral surface2 a of the supporting substrate 2 on the side of the organic adhesivelayer 4 may be more than 0° but less than 180° (see the fragmentarysectional view of the composite substrate in FIG. 2). The angle θ ismore preferably more than 90° but less than 180°, because at this rangethe angle between the top of the piezoelectric substrate 1 and theperipheral surface of the piezoelectric substrate 1 is an obtuse angle,and the same effect as beveling can be obtained. The angle θ may be morethan 0° but less than 90°. In this case, the edge between the top of thepiezoelectric substrate 1 and the peripheral surface of thepiezoelectric substrate 1 may be beveled.

In a method for manufacturing a composite substrate according to thepresent invention described above, it is preferable that the grinding ofa peripheral surface of the laminated substrate 6 in the step (c) isperformed, as illustrated in a fragmentary sectional view of FIG. 3,such that a portion 2 c rising from a plane on the side of the organicadhesive layer 4 in a portion 2 b of the supporting substrate 2 havingthe initial outer diameter of the supporting substrate 2 to theperipheral surface on the side of the organic adhesive layer 4 has acurved surface. If there is a line of intersection between a plane onthe side of the organic adhesive layer 4 in a portion 2 b of thesupporting substrate 2 having the initial outer diameter and theperipheral surface of the supporting substrate 2 on the side of theorganic adhesive layer 4, in lapping and polishing of the peripheralsurface of the laminated substrate 6 in the step (c) described above, astress placed on the line of intersection may cause chipping of thesupporting substrate 2. Grinding the supporting substrate such that therising portion 2 c has a curved surface as illustrated in FIG. 3 caneliminate the line of intersection and thereby prevent chipping of thesupporting substrate 2.

A composite substrate according to the present invention includes asupporting substrate, a piezoelectric substrate, and an organic adhesivelayer for bonding the piezoelectric substrate to the supportingsubstrate. A peripheral surface of the piezoelectric substrate, aperipheral surface of the organic adhesive layer, and a peripheralsurface of the supporting substrate on the side of the organic adhesivelayer are flush with each other. The supporting substrate has an outerdiameter larger than the outer diameter of the peripheral surface on theside of the organic adhesive layer. Such a composite substrate can bemanufactured, for example, by the above-mentioned method formanufacturing a composite substrate.

EXAMPLES Example 1

FIG. 4 shows schematic cross-sectional views of a manufacturing processof a composite substrate according to the present example. First, alithium tantalate substrate (an LT substrate) 10 that had an orientationflat portion (an OF portion) and had a diameter of 100 mm and athickness of 250 μl was prepared as a piezoelectric substrate. A siliconsubstrate 12 that had an OF portion and had a diameter of 100 mm and athickness of 350 μm was prepared as a supporting substrate (FIG. 4( a)).The LT substrate 10 was a 36° Y-cut X-propagation LT substrate, in whichX denotes the propagation direction of a surface acoustic wave (SAW),and 36° Y-cut denotes the cut angle of the substrate rotated about theX-axis with a rotational angle 36° from the Y-axis toward Z-axis. The LTsubstrate 10 and the silicon substrate 12 had a beveled edge. FIG. 5 isa fragmentary sectional view illustrating a bevel. As illustrated in thefigure, the bevel extended from 300 μm inside a peripheral surface ofthe LT substrate 10 and had an angle of 20° at this position. An epoxyadhesive 13 was then applied to the silicon substrate 12 by spincoating. The LT substrate 10 was attached to the silicon substrate 12and was heated to 120° C. Thus, a laminated substrate 16, which includedan organic adhesive layer 14 (a layer formed by solidification of theepoxy adhesive 13) having a thickness of 0.3 μm, was formed (FIG. 4(b)).

A peripheral surface of the laminated substrate 16 was then ground witha grinding apparatus (FIG. 4( c)). A grinding mechanism 20 of thegrinding apparatus used will be described below. FIG. 6 is anexplanatory drawing of the grinding mechanism 20. The grinding mechanism20 is a mechanism in which a grinding wheel 24 at a previously adjustedheight is rotated and moved horizontally to press a whetstone 28 of thegrinding wheel 24 against a peripheral surface of the laminatedsubstrate 16, which is held on a rotating holder 22 by vacuum suctionwith the silicon substrate 12 facing downward and is rotated in the samedirection as the grinding wheel 24, to grind the peripheral surface. Thewhetstone 28 is a circular member having a rectangular cross section andis fixed to an upper side surface of the main body 26 of the grindingwheel. In the present example, the height of the grinding wheel 24 wasadjusted such that the position of the undersurface of the whetstone 28corresponded to the position 100 μm under the top of the siliconsubstrate 12. FIG. 7 illustrates the horizontal movement of the grindingwheel 24 in grinding of the peripheral surface of the laminatedsubstrate 16. FIG. 7 is an A-A′ fragmentary sectional view illustratingthe horizontal movement of the grinding wheel 24 (for the section, seeFIG. 6). A dotted line in the figure indicates the position of thegrinding wheel 24 when the grinding wheel 24 is moved horizontally tofinish grinding. After grinding, as illustrated in FIG. 4( c), thesilicon substrate 12 was ground by 100 μm from the top and 1 mm from theperiphery but had the initial outer diameter. The LT substrate 10 andthe organic adhesive layer 14 were ground up to 1 mm inside theperiphery of the silicon substrate 12. Thus, after grinding, a bevel ofthe LT substrate 10 was removed, and the peripheral surface of the LTsubstrate 10, the peripheral surface of the organic adhesive layer 14,and the peripheral surface of the silicon substrate 12 on the side ofthe organic adhesive layer were flush with each other.

The LT substrate 10 was then lapped and polished with a lapper andpolisher to a thickness of 30 μm (FIG. 4( d)). As the lapper andpolisher, a lapper and polisher that could reduce the thickness andperforming mirror polishing was used, as described below. Morespecifically, to reduce the thickness, a lapper and polisher was usedsuch that the laminated substrate 16 after grinding of the peripheralsurface of the laminated substrate 16 (a ground substrate) was placedbetween a lapping plate and a pressure plate, slurry containing abrasivegrains was supplied between the ground substrate and the lapping plate,and the pressure plate was rotated while the ground substrate waspressed against the surface of the lapping plate by the pressure plate.Subsequently, to perform mirror polishing, a polisher was used such thatthe lapping plate was replaced with a lapping plate having a pad on thesurface, the abrasive grains were replaced with abrasive grains having ahigher grit number, and the pressure plate was rotated and revolved toperform mirror polishing of the surface of the piezoelectric substrate.The surface of the LT substrate that was a ground substrate was firstpressed against the surface of the lapping plate and was polished at arotation speed of 100 rpm for a polishing time of 60 min. The lappingplate was then replaced with a lapping plate having a pad on thesurface, and the abrasive grains were replaced with abrasive grainshaving a higher grit number. Mirror polishing was performed underconditions where the pressure at which the ground substrate was pressedagainst the surface of the lapping plate was 0.2 MPa, the rotation speedwas 100 rpm, the orbital speed was 100 rpm, and the polishing time was60 min.

FIG. 8 illustrates chipping of an edge of the LT substrate 10 when fivecomposite substrates were manufactured by this manufacturing process.FIG. 8 shows freehand drawings prepared by visually inspecting thechipping of an edge of the LT substrate 10. The figure shows that firstto fifth composite substrates had no chipping.

Comparative Example 1

A composite substrate was fabricated in the same way as in Example 1except that, after the formation of the laminated substrate 16illustrated in FIG. 4( b), the surface of the LT substrate 10 was lappedand polished with a lapper and polisher without grinding a peripheralsurface of the laminated substrate 16. FIG. 9 illustrates chipping of anedge of the LT substrate 10 when five composite substrates weremanufactured by this manufacturing process. FIG. 9 shows freehanddrawings prepared by visually inspecting the chipping of an edge of theLT substrate 10. In the drawings, shaded areas indicate chips of the LTsubstrate 10. The figure shows that first to fifth composite substrateshad a plurality of chips.

Example 2

A hundred composite substrates were fabricated in the same way as inExample 1 except that the LT substrate 10 was a 42° Y-cut X-propagationLT substrate, that the thickness of the silicon substrate 12 was 250 μm,that the thickness of the organic adhesive layer 14 was 0.6 μm, and thatthe peripheral surface of the laminated substrate 16 was ground afterthe height of the grinding wheel 24 was adjusted such that the positionof the undersurface of the whetstone 28 corresponded to the position 50μm under the top of the silicon substrate 12. In all of the 100composite substrates thus fabricated, the LT substrate 10 had no chip.

Comparative Example 2

Fifty composite substrates were fabricated in the same way as in Example2 except that the thickness of the silicon substrate 12 was 220 μm andthat the surface of the LT substrate 10 was lapped and polished withoutgrinding the peripheral surface of the laminated substrate 16. In 40 ofthe 50 composite substrates thus fabricated, the LT substrate 10 had achip.

Example 3

A hundred composite substrates were fabricated in the same way as inExample 1 except that the supporting substrate was formed ofborosilicate glass. In all of the 100 composite substrates thusfabricated, the LT substrate 10 had no chip.

Comparative Example 3

Fifty composite substrates were fabricated in the same way as in Example3 except that the surface of the LT substrate 10 was lapped and polishedwithout grinding the peripheral surface of the laminated substrate 16.In 35 of the 50 composite substrates thus fabricated, the LT substrate10 had a chip.

Example 4

Fifty composite substrates were fabricated in the same way as in Example1 except that the piezoelectric substrate was a lithium niobatesubstrate (64° X-propagation LN substrate). In all of the 50 compositesubstrates thus fabricated, the LN substrate had no chip.

Comparative Example 4

Fifty composite substrates were fabricated in the same way as in Example4 except that the surface of the LN substrate was lapped and polishedwithout grinding the peripheral surface of the laminated substrate 16.In 30 of the 50 composite substrates thus fabricated, the LN substratehad a chip.

The results of Examples 1, 2, 3, and 4 and Comparative Examples 1, 2, 3,and 4 showed that the occurrence of chipping of the LT substrate 10 andthe LN substrate during lapping and polishing was lower in lapping andpolishing of the LT substrate 10 and the LN substrate after grinding ofthe peripheral surface of the laminated substrate 16 than in lapping andpolishing of the LT substrate 10 and the LN substrate without grindingthe peripheral surface of the laminated substrate 16.

Example 5

A hundred of the same composite substrates as in Example 1 werefabricated. A surface of the LT substrate 10 of the composite substratethus fabricated was observed with a stereoscopic microscope. As aresult, no organic adhesive layer 14 was deposited on the surface.

Comparative Example 5

Fifty composite substrates were fabricated in the same way as in Example1 except that, after the formation of the laminated substrate 16illustrated in FIG. 4( b), a peripheral surface of the LT substrate 10was ground by 1 mm from the periphery without grinding a peripheralsurface of the silicon substrate 12 of the laminated substrate 16 andthe surface of the LT substrate 10 was lapped and polished with a lapperand polisher. A surface of the LT substrate 10 of the compositesubstrate thus fabricated was observed with a stereoscopic microscope.As a result, in all of the 50 composite substrates thus fabricated, partof the organic adhesive layer 14 was deposited.

The results of Example 5 and Comparative Example 5 showed that grindingthe peripheral surface of the laminated substrate 16 such that theperipheral surface of the LT substrate 10, the peripheral surface of theorganic adhesive layer 14, and the peripheral surface of the siliconsubstrate 12 on the side of the organic adhesive layer 14 were madeflush with each other could prevent the organic adhesive layer 14 frombeing detached and attaching as a contaminant to the surface of the LTsubstrate 10 in the lapping and polishing step, as compared withgrinding the peripheral surface of the LT substrate 10 without grindingthe peripheral surface of the silicon substrate 12. This is probably dueto the following reason. In Comparative Example 5, the peripheralsurface of the LT substrate 10 was ground without grinding theperipheral surface of the silicon substrate 12. Therefore, part of theperipheral surface of the organic adhesive layer 14 was ground, and partof the organic adhesive layer 14 remained outside the peripheral surfaceof the LT substrate 10 after grinding (See the cross-sectional viewafter the grinding of the peripheral surface of the laminated substrate16 illustrated in FIG. 10. A portion within a dotted line indicates aportion that has been ground). In lapping and polishing of the surfaceof the LT substrate 10, the remaining portion of the organic adhesivelayer 14 was probably detached and attached to the surface of the LTsubstrate 10. Contamination of the surface of the LT substrate 10unfavorably results in a decrease in yield, for example, when electrodesare formed on the surface of the LT substrate 10 to fabricate a SAWfilter. In Example 5, in grinding of the peripheral surface of thelaminated substrate 16, the silicon substrate 12 was also ground. Thisensures the removal of a portion of the organic adhesive layer 14outside the peripheral surface of the LT substrate 10. This can preventthe organic adhesive layer 14 from being detached and attaching to thesurface of the LT substrate 10.

Example 6

A composite substrate was fabricated in the same way as in Example 1except that the thickness of the organic adhesive layer 14 was alteredas shown in Table 1. In the composite substrate thus fabricated, aninput electrode and an output electrode each formed of metallic aluminumwere formed on the surface of the LT substrate to fabricate a SAWfilter. The thermal expansion coefficient and the frequency-temperaturecharacteristics of the SAW filter were determined. Table 1 shows themeasurements. The LT substrate has a thermal expansion coefficient inthe SAW propagation direction X of 16 ppm/° C. A single-crystal siliconsubstrate has a thermal expansion coefficient in the SAW propagationdirection X of 3 ppm/° C. The results in Table 1 clearly showed that thefrequency-temperature characteristics (temperature characteristics) werecritically and markedly improved at a thickness of the organic adhesivelayer in the range of 0.1 to 1.0 μm.

TABLE 1 Thickness of the organic adhesive layer(μm) 0.05 0.1 1 2 5 10 15Thermal expansion 16 8 9 16 16 16 16 coefficient (ppm/° C.) Temperature−30 −15 −17 −30 −30 −30 −30 characteristics (ppm/° C.)

The present application claims priority from the Japanese PatentApplication No. 2008-326878 filed on Dec. 24, 2008, the entire contentsof which are incorporated herein by reference.

1. A composite substrate, comprising: a circular supporting substratehaving an orientation flat portion; a piezoelectric substrate; and anorganic adhesive layer for bonding the piezoelectric substrate to thesupporting substrate; wherein a peripheral surface of the piezoelectricsubstrate, a peripheral surface of the organic adhesive layer, and aperipheral surface of the supporting substrate on the side of theorganic adhesive layer are flush with each other; and wherein thesupporting substrate has an outer diameter that is larger than an outerdiameter of the peripheral surface of the supporting substrate on theside of the organic adhesive layer.
 2. The composite substrate accordingto claim 1, wherein the supporting substrate has a smaller thermalexpansion coefficient than the piezoelectric substrate, and the organicadhesive layer has a thickness in the range of 0.1 to 1.0 μm.